Recently, the reduction of emissions and fuel consumption resulted in the further development of hybrid drives for motor vehicles. The aim is to operate the combustion engine in the range of favorable efficiency factors, to switch it off when the vehicle is at a standstill or at low vehicle speeds, and to drive electrically and to utilize braking energy by recuperation. For example, in parallel hybrids the torques of the combustion engine and of one or multiple electric machines are added. The electric machines are connected, e.g., as starter generators to the belt drive or to the crankshaft of the combustion engine.
Rapid load changes or switching operations may induce a bucking movement in the vehicle, in which typically the engine's rotating mass together with the transmission's rotating mass swings against the reduced vehicle mass. In addition, other forms of oscillation are possible as well. Conventional methods for reducing bucking oscillations are based on avoiding the excitation of the drive train by rapid load changes. For this purpose, in the event of rapid changes, the torque requested by the driver via the accelerator pedal is low-pass filtered by reference-forming elements or its rate of change is limited. This causes a delay in torque generation and torque reduction.
In addition, measures are taken in the zero crossing of the drive torque, e.g., in the transition from the overrun state to the acceleration state. The associated zero crossing of the reaction torque causes the engine-transmission unit to tip in the bearings. In addition, mechanical plays or slacks existing in the drive train are traversed. For reasons of comfort, this transition should be “smooth”, which is achieved by a gradient limitation of the drive torque during its passage through zero. This is also the task of the reference-forming element.
For a better understanding of the present invention, the related art is shown in FIG. 1. In hybrid drives, multiple power units, usually one combustion engine and one or more electric machines, together form the drive torque and must be coordinated accordingly by reference-forming element 10′. In the parallel hybrid having a crankshaft starter generator (the flywheel of the combustion engine is coupled with the electric machine, i.e. the crankshaft starter generator), a tipping of the engine-transmission unit is caused by the zero crossing of the combined torque trq of the combustion engine and the electric machine. Mechanical slacks are also traversed in the process. Thus, reference-forming element 10′ would be able to filter the combined setpoint torque trqDes prior to the actual operating strategy, cf. FIG. 1. The distribution of the filtered combined setpoint torque trqDesFlt to the setpoint torques trqLeadEng, trqDesEng of the combustion engine and trgDesElm of the electric machine would then occur, in light of energetic and emission considerations, in operating strategy block 12′.
Modern Otto engines having a manifold injection system usually have an electronic throttle valve for regulating the air mass flow. The accelerator pedal is mechanically decoupled from the electronic throttle valve. The finite adjusting speed of the throttle valve actuator and dynamic filling effects in the intake manifold do not allow for a highly dynamic adjustment of a specified air mass flow and of the combustion engine torque produced thereby. An intervention in the ignition angle and an associated reduction of the combustion engine torque, by contrast, may occur nearly without delay. Thus two paths are available for controlling the torque. For each of these two paths, an associated setpoint torque may be specified.
The lead setpoint torque trqLeadEng for the combustion engine, shown in FIG. 1, acts on the slow air path of the combustion engine. The air mass flow is adjusted accordingly. At an optimum ignition angle, the combustion engine would produce a torque trqBs known as a base torque. In stationary operation, base torque trqBs approximately corresponds to lead setpoint torque trqLeadEng. In non-stationary operation, dynamic filling effects are active in the intake manifold, the transmission of lead setpoint torque trqLeadEng onto base torque trqBs being approximately describable using the series connection of a dead-time element (TO) and a time-delay element of the first order (Ptl).
The second setpoint torque trqDesEng shown in FIG. 1 for the combustion engine acts on the fast ignition angle path. An ignition retard of the ignition angle with respect to the optimum ignition angle deteriorates the efficiency factor of the engine, and the actual torque trqEng of the combustion engine is reduced with respect to base torque trqBs.
The model in FIG. 1 shows the correlations in a simplified manner. The setpoint torque for the combustion engine trqDesEng is limited to a range between base torque trqBs and a minimum base torque trqBsMin and yields the actual torque trqEng of the combustion engine. The time delay in the ignition angle intervention is small and is disregarded in the model. Base torque trqBs corresponds to the actual torque trqEng of the combustion engine, which results at the optimum ignition angle. Minimum base torque trqBsMin is lower than the base torque and corresponds to the actual torque trqEng in the event of a maximum ignition retard.
In the case of a diesel engine, in particular a turbocharged diesel engine, the torque control may also be divided into a slow path having a lead setpoint torque trqLeadEng and a fast path having a setpoint torque trqDesEng, cf. DE19630213 C1.
The engine control system of a modern combustion engine is able to ascertain the current actual torque trqEng and the base torque trqBs on the basis of measured or estimated variables, in a gasoline engine having manifold injection, e.g., from the engine speed, the intake manifold pressure, ignition timing and air ratio λ.
The torque control system of modern electric machines has a much higher dynamics in comparison to the slow path of the torque control system of a combustion engine. In the model shown in FIG. 1, the delay in the torque control of the electric machine is neglected, the actual torque trqElm resulting from setpoint torque trqDesElm by limitation to torque limits trqElmMax and trqElmMin. The torque limits of the electric machine are ascertained on the basis of the current operating states of the electric machine, of one or more electrical energy stores and one or more vehicle electrical systems.
The method of reference formation shown in FIG. 1 works as long as both power units have a similar behavior in the dynamics of the torque control. This is the case on account of the high dynamics of the electric machine if the torque of the combustion engine is influenced via the fast path trqDesEng.
If the torque of the combustion engine is determined by the slow path, e.g., in the case of rapidly rising setpoint torques and a delayed rising base torque trqBs, which limits the actual torque trqEng (i.e. trqDesEng>trqBs), then the following problems result:                1. The dynamics of the actual torque trqEng and thus of the combined actual torque trq are reduced due to the series connection of two delays, namely, the filtering process in reference-forming element 10′ and the slow path in the combustion engine (intake manifold dynamics, turbo lag). The dynamics of the slow path depend heavily on the operating point of the combustion engine, i.e., torque, rotational speed, intake manifold pressure etc., such that a consideration/compensation in operating strategy block 12′ would be possible only in a limited way.        2. Due to the varying dynamics in the torque control of the two power units, combined actual torque trq greatly deviates in non-stationary operation from filtered combined setpoint torque trqDesFlt. In addition, the deviation varies depending on the currently chosen torque distribution (operating strategy). A limitation of the gradient of trqDesFlt in the zero crossing then does not result in the limitation of the gradient of trq in its zero crossing, which affects the driving comfort negatively.        